Invisible Portlet in Liferay Portal

Heyho community,

I recently started dealing with developing Vaadin portlets and their integration into Liferay portal.
My very first approach on these issues was to just start a new Vaadin project in eclipse and use the created “Hello Vaadin user.” portlet without making any changes to the code.
This works perfectly.
My second approach then was to deploy the simple address book application from the Vaadin tutorials as a portlet in Liferay portal. That’s where I’m stuck at the moment.
The address book portlet works fine for the administrator user of Liferay portal, but no other user of the portal is able to see the address book portlet, although the permissions are all set correctly as far as I can see.

I’m using Liferay portal v6.1.1 and the portlets are based on Vaadin v6.7.6.

Any help is highly appreciated.